A cylinder bore surface for an engine secures a predetermined level of surface roughness through a honing processing. On the honing processed cylinder bore surface, a network-shaped mesh pattern is formed, and an oil film is formed on the bore surface by such a mesh pattern to perform a function of minimizing a friction when performing a piston movement.
When a piston movement is continuously performed, a mesh pattern may be deformed, such deformation causes an oil to separate from the cylinder bore surface when performing a piston movement and thus a friction increases between a piston ring and the cylinder bore surface, thereby largely deteriorating engine efficiency.
The above-described problem may be solved by additionally forming an oil groove (e.g., 10 μm or less) that is charged with oil on the honing processed cylinder bore surface. A method of processing an oil groove that is formed on the cylinder bore surface is generally classified into a laser processing method and a mechanical processing method.
The laser processing method is a method of processing an oil groove on the cylinder bore surface using pulse laser after a honing processing, and in such a laser processing method, a processing equipment is expensive and because a size and volume thereof are large, there is a problem that equipment installation is not easily, and there is a problem that it is very difficult to process the oil groove in a constant depth because of a micro output change of laser beam.
The mechanical processing method is a method of physically forming an oil groove on the cylinder bore surface by inserting a cutting tool into the honing processed cylinder bore and contacting the cutting tool with the honing processed cylinder bore. When using such a mechanical processing method, in order to form a groove of a constant depth on an entire surface of the cylinder bore, it is required to accurately correspond a rotation axis of the cutting tool with a central axis of the cylinder bore.
For the above-described axis position setting, a measurement equipment is introduced and a measurement process should be performed. This becomes a factor that increases an entire processing time. Further, even if an axis position is accurately measured with the measurement equipment and a processing is performed, a depth (10 μm or less) of an oil groove that should be formed is very minute and thus when measurement is minutely incorrect, there is a problem that a depth of the oil groove that is formed on an entire surface of the cylinder bore is not constantly formed.